Filesystem Hierarchy Standard

/var contains variable data files. This includes spool
directories and files, administrative and logging data, and transient
and temporary files.

Some portions of /var are not shareable between different
systems. For instance, /var/log, /var/lock, and
/var/run. Other portions may be shared, notably
/var/mail, /var/cache/man, /var/cache/fonts,
and /var/spool/news.

/var is specified here in order to make it possible to mount
/usr read-only. Everything that once went into /usr
that is written to during system operation (as opposed to installation
and software maintenance) must be in /var.

If /var cannot be made a separate partition, it is often
preferable to move /var out of the root partition and into the
/usr partition. (This is sometimes done to reduce the size of
the root partition or when space runs low in the root partition.)
However, /var must not be linked to /usr because this
makes separation of /usr and /var more difficult and is
likely to create a naming conflict. Instead, link /var to
/usr/var.

Applications must generally not add directories to the top level of
/var. Such directories should only be added if they have some
system-wide implication, and in consultation with the FHS mailing list.